System and method for generating beams of light using an anisotropic acousto-optic modulator

ABSTRACT

A light source system includes a beam source generating a first input beam of light with first and second beam components. The first component has a first linear polarization and a first frequency. The second component has a second linear polarization and a second frequency. The first and second linear polarizations are orthogonal. An anisotropic acousto-optic modulator (AOM) is positioned to receive the first input beam. The AOM is operable to change the polarization and frequency of the first and the second beam components in response to a control signal, and thereby generate first and second output beams corresponding to the first and second components, respectively.

BACKGROUND

Measurement optics in a polarization-based or multiplexed heterodyneinterferometer such as used for precision measurements in semiconductordevice manufacturing equipment generally use a light beam includingorthogonal polarization components that have different frequencies. Inheterodyne interferometry, a dual frequency/dual polarization source oflight is used. The frequency difference between the two orthogonallypolarized beam components is important because it can be the limit tohow fast something can move and the distance still be measuredaccurately by this type of measurement system. Zeeman split HeNe laserscan provide orthogonally polarized light components, but the differencefrequency is limited to a maximum of about 8 MHz. A two-mode frequencystabilized HeNe laser can also provide two orthogonally polarized beamswith frequency separation, but this frequency difference is in the500-1500 MHz range and cannot be easily utilized by the processingelectronics. The desired frequency range that will fulfill thelithography industries need for speed, but is compatible with currentelectronic technology is about 7-30 MHz.

Several methods of producing a desired frequency split in a heterodyneinterferometer have been used in the past. Most of these prior solutionsinvolve conditioning the light to get the desired frequency after thestabilized laser source. One prior solution is to use two high frequencyacousto-optic modulators (AOMs) to generate the desired differencefrequency. The laser source beam is split into two beams of orthogonalpolarization. Each linearly polarized beam is sent through an AOM. Thefirst order diffracted beams from each AOM are redirected using mirrorsand recombined using a second beam splitter to become collinear andco-bore again. While the absolute frequency of the AOMs in this priorsolution is typically too high to be ideal (e.g., 80 MHz) the differencein frequency between the two different AOMs can be adjusted (e.g., oneat 80 MHz and the other at 90 MHz) so that when the two orthogonal,linearly polarized beam components are recombined, they have the desireddifference frequency. Unfortunately, this is a more costly solution,because two AOMs are used to achieve the desired results (along with abeam splitter, two turning mirrors and a second beam splitter which actsas a beam recombiner). Other solutions using two AOMs exist, but allhave the disadvantage of multiple components (e.g., minimum of two AOMunits and a beam splitter), which tends to increase the cost of thesolutions.

Another prior approach is to use a single low frequency isotropic AOMwith a single acoustic wave and a birefringent recombination prism.While this method reduces the number of components as compared to thepreviously described two-AOM solution, it has significant issues of itsown. The major disadvantages include: a significant portion of thesource light is discarded, (even with a single polarization outputlaser); the solution takes a lot of space to accomplish; and thesolution does not fully accomplish a secondary benefit of AOM frequencyshifters in providing isolation for the laser because it only isolatesfeedback on one polarization. In this prior method, only a singlepolarization and frequency are desired prior to the AOM device, so for aZeeman split HeNe laser, a polarizer is typically used to filter out theother polarization/frequency component from the source laser. Thus, halfthe source light is eliminated before the beam enters the AOM.

In the isotropic acoustic wave interaction of this prior solution, thereis no effect on the beam's polarization, so the diffracted (firstorder), frequency shifted beam is the same polarization as the zeroorder or un-diffracted beam. Exiting the AOM, the zero order and firstorder beams have a frequency difference of around 20 MHz in a currentdevice on the market. The job of making the beams collinear again isaccomplished by passing the beams through a birefringent recombinationprism. The beam separation angle exiting this type of AOM is small, sono compensation is made for making the beams co-bore again after theyare made parallel with the recombination prism. Typically, the opticaxis of the recombination prism is at a forty-five degree angle to thepolarization of the beams. The recombination prism splits each beam intotwo orthogonally polarized components. One component sees the index ofrefraction of n_(e) and the other component sees the index of refractionof n_(o). The two beams refract differently at the entrance and exitprism/air interfaces due to this index difference. The apex angle of theprism is optimized to allow one polarization component of each beam tobecome parallel again. The other two unwanted polarized beams exitingthe recombination prism are not parallel to the desired beams and areapertured. This recombination scheme effectively throws away half theoptical power in the first and zero order beams. The net result is thatthree-fourths of the original source optical power for a Zeeman splitlaser (more if the AOM device operates in the Raman Nath regime) is lostusing this prior single isotropic AOM method of increasing the frequencysplit.

It is desirable to have a small footprint or package for a heterodyneinterferometry light source, as this light source is often installed ina customer's equipment. The single low-frequency isotropic AOM solutionhas issues that demand more space than desired. To get adequateefficiency for a low frequency isotropic AOM, a long interaction lengthis necessary, so the device itself is quite long. Also, the separationangle between the diffracted orders on this device is small, so a longdistance is typically used to get adequate beam separation to apertureoff the unwanted beams following the recombination prism. Thus, a longfootprint, additional optics to focus the light to a pinhole spatialfilter, or additional optics to fold the beam path in the package may beused to address this issue.

In addition, when using a single low frequency isotropic AOM with zeroand first order beams, the zero order beam does not protect the laserfrom feedback because the frequency in that path is still the laserfrequency (not shifted up or down). Reflections from this beam upstreamthat make it back to the laser will cause wavelength stability problemsand a possible loss of lock for the laser.

In another prior approach, two shifted frequency beams are generated inthe same isotropic AOM. The frequency shifts for both beams areaccomplished in a couple of different ways. The first is to use oneacoustic wave in the AOM. There is a polarizing beam splitter before (orattached to the AOM) to split a single frequency polarized beam into twoorthogonally polarized beams. The polarizing beam splitter also does thetask of orienting the two orthogonally polarized beams at the plus andminus Bragg angle of the AOM device so that one beam is up-shifted andone beam is down-shifted by the single acoustic wave frequency. The AOMitself is isotropic and does not affect the polarization of the beams.The frequency difference between the output beams is two times the AOMfrequency. In another form of the single isotropic AOM solution, alonger crystal is used, and each polarized beam traverses through twoacoustic waves in series, which are generated by two transducers of theAOM. The net result on the output beams is a frequency difference of twotimes the difference in frequency of the two AOM transducers. Again,this is an isotropic interaction (i.e., it does not affectpolarization), and a beam splitter is used before the AOM device togenerate two beams of orthogonal polarization and moving in divergingdirections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram illustrating a system for producing frequency-shiftedbeams with orthogonal linear polarizations in a first embodimentaccording to the present teachings.

FIG. 2 is a diagram illustrating a system for producingfrequency-shifted beams with orthogonal linear polarizations in a secondembodiment according to the present teachings.

FIG. 3 is a diagram illustrating a system for producingfrequency-shifted beams with orthogonal linear polarizations in a thirdembodiment according to the present teachings.

FIG. 4 is a diagram illustrating the combining of beams in a firstembodiment according to the present teachings.

FIG. 5 is a diagram illustrating the combining of beams in a secondembodiment according to the present teachings.

FIG. 6 is a block diagram illustrating an interferometer system in oneembodiment according to the present teachings.

FIG. 7 is a diagram illustrating a system for producingfrequency-shifted beams with orthogonal linear polarizations in a fourthembodiment according to the present teachings.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownspecific and illustrative embodiments according to the presentteachings. It is to be understood that other embodiments may be utilizedand structural or logical changes may be made without departing from thescope of the appended claims. The following Detailed Description,therefore, is not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims.

FIG. 1 is diagram illustrating a system 100A for producingfrequency-shifted beams with orthogonal linear polarizations in a firstembodiment according to the present teachings. System 100A includeslaser light source (laser) 102 and acousto-optic modulator (AOM) 108.Laser 102 acts as a source of a heterodyne beam 104 having two distinctfrequency components (f1 and f2) with orthogonal linear polarizations(e.g., horizontal and vertical). A beam or beam component with avertical polarization, such as beam component 106A, is represented inthe Figures by an upward and downward pointing arrow, and a beam or beamcomponent with a horizontal polarization, such as beam component 106B,is represented by a circle. An exemplary embodiment of laser 102 is acommercially available He—Ne laser such as a Model 5517B available fromAgilent Technologies, Inc., which uses Zeeman splitting to generate thetwo frequency components in the same laser cavity. Zeeman splitting inthis manner can generate a laser beam having frequency components withfrequencies f1 and f2 and a frequency difference (f2−f1) of about 2 MHz.The two frequency components f1 and f2 have opposite circularpolarizations, and a quarter-wave plate is used to change thepolarizations of the frequency components so that the two frequencycomponents have orthogonal linear polarizations. In another specificembodiment, laser 102 is a two-mode frequency stabilized laser. A Zeemanlaser has better frequency stability than a laser using the two-modefrequency stabilization method.

In the illustrated embodiment, AOM 108 is an anisotropic, low frequencyshear wave AOM with a TeO₂ uniaxial crystal. One example of an AOMdevice that is suitable for use in implementing AOM 108 is the FS1102AOM produced by Isomet Corporation (www.isomet.com), having headquarterslocated at 5263 Port Royal Road, Springfield, Va. 22151. AOM 108includes electro-acoustic transducers 110 for receiving control signals.The electro-acoustic transducers 110 convert electrical signals intosound waves that are launched through the crystal of the AOM 108. Thetransducers 110 excite the AOM 108 with two acoustic waves of the sameor different frequencies, and with a small angle between theirpropagation directions (i.e., the two waves have different propagationvectors, which are identified in FIG. 1 by K1 and K2). Two acousticwaves or beams are used so that correct phase matching can exist forboth input beam components 106A and 106B. A first one of the acousticwaves acts on the horizontal polarization 106B of the orthogonallypolarized laser source beam 104, and a second one of the acoustic wavesis phase matched to the vertical polarization 106A of the source beam104.

For a laser beam 104 with a given propagation direction in the crystalof the AOM 108, the laser field can be decomposed into two componentsaccording to the polarization. One of the components is call theordinary wave while the other is called the extraordinary wave. Thepropagation speed of the ordinary wave is different from the propagationspeed of the extraordinary wave. If there is no acoustic field in thecrystal of the AOM 108, the ordinary wave and the extraordinary wavepreserve their propagation directions as well as their polarizations.The propagation directions of the laser beam 104 and the acoustic fieldsin the crystal of the AOM 108 are chosen so that the extraordinary wavein the input laser beam 104 is phase matched to be down-converted(diffracted) into the ordinary wave by one of the acoustic fields.Simultaneously, the ordinary wave in the input laser beam 104 is phasematched to be up-converted (diffracted) into the extraordinary wave bythe other acoustic field.

AOM 108 is operated in a low-frequency shear mode. AOM 108 diffracts thetwo input beam components 106A and 106B in opposite directions, therebyproducing plus first order beam 114A, which corresponds to component106A, and minus first order beam 114B, which corresponds to component106B. AOM 108 causes an increase in the frequency of component 106A, adecrease in the frequency of component 106B, and causes a ninety-degreerotation in the polarization of both components 106A and 106B. The neteffect is that while both input beam components 106A and 106B changepolarization (i.e., horizontal becomes vertical and vertical becomeshorizontal), the beams stay orthogonally polarized, and now have afrequency difference given by the following Equation I:fsplit=(f1+faomtransducer1)−(f2−faomtransducer2)  Equation I

-   -   where:        -   fsplit=difference in the frequency of beam 114A and the            frequency of beam 114B;        -   f1=frequency of beam component 106A;        -   f2=frequency of beam component 106B;        -   faomtransducer1=frequency of first signal provided to            transducers 110; and        -   faomtransducer2=frequency of second signal provided to            transducers 110.

In a specific embodiment according to the present teachings,faomtransducer1 and faomtransducer2 are both in the range of about 10 to450 MHz. In the embodiment shown in FIG. 1, faomtransducer1 is the sameas faomtransducer2, and this common frequency is identified in FIG. 1 bythe term “fAOM”.

As an example, if the transducers 110 are provided with a 10 MHz RFcontrol signal, both output beams 114A and 114B will be frequencyshifted by 10 MHz, but in opposite directions (i.e., plus 10 MHz andminus 10 MHz), for a frequency split or difference of 20 MHz caused bythe AOM 108. If the Zeeman laser light source 102 provides a 2 MHz split(i.e., |f2−f1|=2 MHz), system 100A provides a total frequency split(fsplit) in beams 114A and 114B of 18 MHz or 22 MHz. In a specificembodiment according to the present teachings, the total frequency split(fsplit) in beams 114A and 114B is in the range of 8 to 30 MHz.

Using an anisotropic AOM 108 is optimal for a Zeeman split laser 102,which already has two orthogonal polarizations and frequencies. In theembodiment shown in FIG. 1, a single, low frequency, anisotropic AOM 108is used to increase the frequency difference, |f1−f2|, between inputbeam component 106A and input beam component 106B. The same embodimentcan also be used, with the proper frequency applied to AOM 108, todecrease the frequency difference, |f1−f2|, between input beam component106A and input beam component 106B. In a specific embodiment accordingto the present teachings, beams 114A and 114B have a combined opticalpower that is substantially the same as the optical power of beam 104.

FIG. 2 is a diagram illustrating a system 100B for producingfrequency-shifted beams with orthogonal linear polarizations in a secondembodiment according to the present teachings. In the illustratedembodiment, system 100B includes the same laser light source (laser) 102and acousto-optic modulator (AOM) 108 as system 100A (FIG. 1), and thelaser 102 and AOM 108 shown in FIG. 2 operate in the same manner asdescribed above with respect to FIG. 1. One difference between system100B and system 100A is that system 100B also includes a second AOM 202,which is positioned between the laser 102 and AOM 108. AOM 202 is ahigh-frequency isotropic AOM that up-shifts or down-shifts the frequencyof both components 106A and 106B of the beam 104 by the same amount(e.g., 30-500 MHz).

As described above with respect to FIG. 1, AOM 108 up shifts thefrequency (f1) of the first component 106A of beam 104, and down shiftsthe frequency (f2) of the second component 106B of beam 104. In someapplications, the frequency shifts provided by AOM 108 may not besufficient to isolate the laser 102 from optical feedback. To provideadditional isolation, AOM 202 is added between laser 102 and AOM 108.AOM 202 is an isotropic high-frequency AOM that is used to up shift thefrequency of both components 106A and 106B of the beam 104 by the samerelatively large amount (e.g., 80 MHz) to provide better opticalisolation. Since AOM 202 is isotropic, the polarizations of theorthogonal beam components 106A and 106B are not affected by AOM 202. Ina specific embodiment according to the present teachings, AOM 108 andAOM 202 are pre-aligned in a single package.

AOM 202 generates an output beam 204 with up-shifted frequencycomponents (f1+fAOM1 and f2+fAOM1), where fAOM1 represents the signalfrequency applied to AOM 202 (e.g., 80 MHz). The first up-shiftedfrequency component 206A (f1+fAOM1) has a vertical linear polarization,and the second up-shifted frequency component 206B (f2+fAOM1) has ahorizontal linear polarization. The output beam 204 from AOM 202 isprovided as an input beam to AOM 108.

In the embodiment shown in FIG. 2, faomtransducer1 (Equation I) is thesame as faomtransducer2 (Equation I), and this common frequency isidentified in FIG. 2 by the term “fAOM2”. AOM 108 up-shifts thefrequency of the first component 206A (f1+fAOM1) of beam 204 by anamount fAOM2, and changes the polarization of the first component fromvertical to horizontal, resulting in a horizontally polarized beam 212Athat has a frequency of f1+fAOM1+fAOM2. Similarly, AOM 108 down-shiftsthe frequency of the second component 206B (f2+fAOM1) of beam 204 by anamount fAOM2, and changes the polarization of the second component fromhorizontal to vertical, resulting in a vertically polarized beam 212Bthat has a frequency of f2+fAOM1−fAOM2.

In the embodiment shown in FIG. 2, beam 212A is coupled into opticalfiber 216A by lens 214A, and beam 212B is coupled into optical fiber216B by lens 214B. Optical fibers 216A and 216B carry beams 212A and212B downstream to a beam-combining unit that combines the beams 212Aand 212B into a combined beam for use in interferometer optics at ameasurement site. In a specific embodiment according to the presentteachings, fibers 216A and 216B are polarization-maintaining (PM)fibers.

The use of fibers 216A and 216B allows the laser 102 and AOMs 108 and202 to be positioned remotely from the interferometer optics so that thelaser 102 and the AOMs 108 and 202 do not affect the thermal environmentof the interferometer optics. Sending the separate beams 212A and 212Bon corresponding separate fibers 216A and 216B avoids cross-talk betweenthe polarization components. The use of fibers 216A and 216B to deliverthe light downstream provides several other advantages, including: (1)compensation for pointing stability issues caused by ambient temperaturevariations is not necessary when the light is delivered with an opticalfiber; (2) there is no need for additional optics to make the beams 216Aand 216B co-bore, and the co-linearity specification is much looser; and(3) fiber delivery combined with the increased split frequency providedby system 100B reduces or eliminates the need for electronics at thedownstream metrology stage area that might generate heat.

In the embodiment shown in FIG. 2, AOM 202 is an isotropic AOM. Inanother embodiment, AOM 202 is an anisotropic AOM that changes thepolarization of both beam components 106A and 106B.

In the embodiment shown in FIG. 2, AOM 202 is positioned between thelaser 102 and AOM 108. In another embodiment, AOM 202 is positionedbetween AOM 108 and the lenses 214A and 214B.

FIG. 3 is a diagram illustrating a system 100C for producingfrequency-shifted beams with orthogonal linear polarizations in a thirdembodiment according to the present teachings. In the illustratedembodiment, system 100C includes the same laser light source (laser) 102and acousto-optic modulators (AOMs) 108 and 202 as system 100B (FIG. 2),and the laser 102 and AOMs 108 and 202 shown in FIG. 3 operate in thesame manner as described above with respect to FIG. 2. One differencebetween system 100C and system 100B is that system 100C includesbirefringent recombination prism or wedge 302 rather than the lenses214A and 214B, and the optical fibers 216A and 216B shown in FIG. 2.

The anisotropic AOM 108 has a larger diffraction angle than the previoussolutions that use a single isotropic AOM, so the co-bore (in additionto the co-linearity) of the output beams 212A and 212B should beaddressed if fiber delivery is not used. In the embodiment shown in FIG.3, the co-linearity angles are adjusted with birefringent recombinationprism 302. Prism 302 receives beams 212A and 212B from AOM 108, andre-directs these beams 212A and 212B to produce corresponding parallelbeams 306A and 306B. The optic axis of the prism 302 is identified at304 in FIG. 3. The two beams 212A and 212B refract differently at theentrance and exit prism/air interfaces, and the prism 302 isappropriately positioned to cause the input beams 212A and 212B tobecome corresponding parallel beams 306A and 306B. The beams 212A and212B exiting the AOM 108 are orthogonally polarized, so very littlelight is lost in the recombination prism 302.

FIG. 4 is a diagram illustrating the combining of parallel beams in afirst embodiment according to the present teachings. As shown in FIG. 4,the parallel beams 306A and 306B produced by prism 302 (FIG. 3) areprovided to lens 402. Lens 402 combines beams 306A and 306B, therebyproducing a combined beam that is directed into a polarizationmaintaining optical fiber 404. Optical fiber 404 carries the combinedbeam downstream to interferometer optics at a measurement site.

FIG. 5 is a diagram illustrating the combining of parallel beams in asecond embodiment according to the present teachings. As shown in FIG.5, the parallel beams 306A and 306B produced by prism 302 (FIG. 3) areprovided to walk off prism 502. Walk off prism 502 “walks” beams 306Aand 306B back together so that they are co-bore, thereby producingcombined beam 504. Combined beam 504 has one component 506A with ahorizontal polarization, and another component 506B with a verticalpolarization. The combined beam 504 is provided to interferometer opticsat a measurement site. Adjusting the tilt of the prism 302 cancompensate for any errors in co-linearity caused by an imperfect walkoff prism.

FIG. 6 is a block diagram illustrating a two-frequency interferometersystem 600 in one embodiment according to the present teachings.Interferometer 600 includes laser light source 102, AOM 108, lenses 602Aand 602B, optical fibers 650 and 655, beam-combining unit 660, analysissystem 680, and interferometer optics 690. Laser 102 and AOM 108 operateas described above with respect to FIG. 1 to producelinearly-orthogonally polarized beams 114A and 114B. Laser 102 usesZeeman splitting to generate a heterodyne beam 104, and anisotropic AOM108 flips the polarization of the two beam components 106A and 106B, andincreases the frequency difference between the two beam components 106Aand 106B, and thereby produces linearly-orthogonally polarized beams114A and 114B. In another embodiment, system 600 is configured as afree-beam system, rather than using fiber delivery as shown in FIG. 6.In yet another embodiment, system 600 includes a second AOM 202positioned between the laser 102 and anisotropic AOM 108 or after AOM108, as shown in FIGS. 2 and 3 and described above.

In the embodiment shown in FIG. 6, lenses 602A and 602B focus the beams114A and 114B into separate polarization-maintaining optical fibers 650and 655, respectively. Polarization-maintaining fibers 650 and 655deliver the beams 114A and 114B to beam-combining unit 660, whichdirects the two beams into a beam combiner 670.

The use of optical fibers 650 and 655 allows laser 102 and AOM 108 to bemounted away from interferometer optics 690. Accordingly, heat generatedin laser 102 and AOM 108 does not disturb the thermal environment ofinterferometer optics 690. Additionally, laser 102 and AOM 108 do notneed to have fixed positions relative to interferometer optics 690,which may provide significant advantages in applications having limitedavailable space near the object 699 being measured.

Beam-combining unit 660 precisely aligns input beam 114A (INR) and inputbeam 114B (INT) from optical fibers 650 and 655 for combination in beamcombiner 670 to form a collinear output beam COut. Beam combiner 670 canbe a coated polarizing beam splitter that is used in reverse. Combinedbeam COut is input to interferometer optics 690. In interferometeroptics 690, a beam splitter 675 reflects a portion of beam COut toanalysis system 680, and analysis system 680 uses the two frequencycomponents of the light reflected in beam splitter 675 as first andsecond reference beams. The remaining portion of combined beam COut canbe expanded in size by a beam expander (not shown) before entering apolarizing beam splitter 692.

Polarizing beam splitter 692 reflects one of the polarizations (i.e.,one frequency beam) to form a third reference beam directed throughoptics 696 toward a reference reflector 698 and transmits the otherlinear polarization (i.e., the other frequency) as a measurement beamthrough optics 694 toward an object 699 being measured. In analternative version of the interferometer optics 690, a polarizing beamsplitter transmits the component that forms the measurement beam andreflects the component that forms the reference beam.

Movement of the object 699 being measured causes a phase change in themeasurement beam that analysis system 680 measures by combining themeasurement beam with the third reference beam to form a beat signal. Toaccurately determine the phase change caused by the movement of theobject 699, the phase of this beat signal can be compared to the phaseof a reference beat signal generated from a combination of the first andsecond reference beams. Analysis system 680 analyzes the phase change todetermine the speed of and/or distance moved by the object 699.

FIG. 7 is a diagram illustrating a system 100D for producingfrequency-shifted beams with orthogonal linear polarizations in a fourthembodiment according to the present teachings. In the illustratedembodiment, system 100D includes the same acousto-optic modulator (AOM)108 as systems 100A-100C, but system 100D uses a different laser 702than the laser 102 of system 100A. In the embodiment shown in FIG. 7,laser 702 acts as a source of a beam 704 having a single frequency (f1)with a single linear polarization. The single linear polarization is a45 degree polarization in the illustrated embodiment, which isrepresented in FIG. 7 by arrow 706.

AOM 108 acts as a polarizing beam splitter and splits the input beam 704into a horizontally polarized beam component and a vertically polarizedbeam component. AOM 108 diffracts these two orthogonally polarized beamcomponents in opposite directions, thereby producing plus first orderbeam 714A, and minus first order beam 714B. AOM 108 causes an increasein the frequency of one beam component, a decrease in the frequency ofthe other beam component, and causes a ninety-degree rotation in thepolarization of both beam components. The net effect is that while bothbeam components change polarization (i.e., horizontal becomes verticaland vertical becomes horizontal), the beams stay orthogonally polarized,and now have a frequency difference given by the following Equation II:fsplit=(f1+faomtransducer1)−(f1−faomtransducer2)  Equation II

-   -   where:        -   fsplit=difference in the frequency of beam 714A and the            frequency of beam 714B;        -   f1=frequency of beam 704; faomtransducer1=frequency of first            signal provided to transducers 110; and        -   faomtransducer2=frequency of second signal provided to            transducers 110.

In a specific embodiment according to the present teachings,faomtransducer1 and faomtransducer2 are both in the range of about 10 to450 MHz. In the embodiment shown in FIG. 7, faomtransducer1 is the sameas faomtransducer2, and this common frequency is identified in FIG. 7 bythe term “fAOM”.

As an example, if the transducers 110 are provided with a 10 MHz RFcontrol signal, both output beams 714A and 714B will be frequencyshifted by 10 MHz, but in opposite directions (i.e., plus 10 MHz andminus 10 MHz), for a frequency split or difference of 20 MHz caused bythe AOM 108. In a specific embodiment according to the presentteachings, the total frequency split (fsplit) in beams 714A and 714B isin the range of 8 to 30 MHz.

In another embodiment according to the present teachings, the input beam704 has a polarization state other than 45 degrees. Regardless of whatpolarization state is chosen for beam 704, when the beam 704 enters thecrystal of AOM 108, the polarization is decomposed into two orthogonaleigen polarizations. The optical power in each eigen polarizationdepends on the polarization state of the input beam 704. Linearpolarization oriented at 45 degrees from the optical axis of the crystalof AOM 108 is used for beam 704 in one form of the invention because itresults in two output beams 714A and 714B with equal optical power. In aspecific embodiment according to the present teachings, beams 714A and714B have a combined optical power that is substantially the same as theoptical power of beam 704.

In another embodiment according to the present teachings, system 100Dincludes a second AOM 202 positioned between the laser 702 andanisotropic AOM 108 or after AOM 108, as shown in FIGS. 2 and 3 anddescribed above. In a specific embodiment according to the presentteachings, interferometer system 600 (FIG. 6) uses a single frequency,single polarization laser, such as laser 702, rather than the twofrequency, two polarization laser 102 shown in FIG. 6.

Specific embodiments according to the present teachings provide severaladvantages over prior solutions. Specific embodiments according to thepresent teachings provide a significant optical power savings, can beimplemented in less space, and have better optical isolation than priorsolutions. In the anisotropic low frequency AOM device 108, theinteraction length is shorter, and the beams exit the AOM 108 with alarger separation angle as compared with the prior solution of using asingle isotropic AOM device. Both of these properties lead to a smaller,more compact package for the final product. The shorter interactionlength means the AOM device 108 can be much smaller. The largerseparation angle means that the unwanted beams can be apertured in ashorter distance. Both beams produced by AOM 108 are shifted infrequency, so AOM 108 is a better optical isolator for the laser 102than the prior solution of using a single isotropic AOM.

Specific embodiments according to the present teachings are less complexthan the prior solutions that use two high frequency AOMs, in that apolarization beam splitter is not used prior to the AOM device 108 todivide a single polarization beam into two orthogonally polarized beamsand alter the direction of the beams. In addition, AOM 108 is furtherdistinguishable over many prior solutions in that AOM 108 uses ananisotropic interaction, rather than the isotropic interaction used inthese previous solutions.

In one prior approach, an anisotropic AOM is used with two acousticfrequencies in series to generate orthogonally polarized frequencyshifted beams from a single polarization optical source. In contrast,specific embodiments according to the present teachings that use thedual acoustic wave AOM 108 do not generate two orthogonal polarizationsfrom a single input polarization. Rather, the AOM 108 preserves the twopolarizations of the laser 102 throughout the device 108, while upshifting one beam and downshifting the other beam.

In other specific embodiments according to the present teachings, AOM108 is configured to generate orthogonally polarized frequency shiftedbeams from a single polarization optical source. Regardless of whetherAOM 108 is used with a single polarization source, or a two polarizationsource, such as a Zeeman laser, the AOM 108 according to a specificembodiment preserves or maintains the optical power of the input beamthat is provided to the AOM 108. Thus, in both of these cases, theoutput beams that exit the AOM 108 have a combined optical power that issubstantially the same as the optical power of the input beam thatenters the AOM 108.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A light source system comprising: a beam source generating a firstinput beam of light with first and second beam components, the firstcomponent having a first linear polarization and a first frequency, thesecond component having a second linear polarization and a secondfrequency, wherein the first and second linear polarizations areorthogonal; and an anisotropic acousto-optic modulator (AOM) positionedto receive the first input beam, wherein the AOM is operable to changethe polarization and frequency of the first and the second beamcomponents in response to a control signal, and thereby generate firstand second output beams corresponding to the first and secondcomponents, respectively.
 2. The light source system of claim 1, whereinthe beam source comprises a laser.
 3. The light source system of claim2, wherein the laser comprises a Zeeman laser.
 4. The light sourcesystem of claim 3, wherein the laser comprises a two-mode frequencystabilized laser.
 5. The light source system of claim 1, wherein thefirst output beam has the second linear polarization, and wherein thesecond output beam has the first linear polarization.
 6. The lightsource system of claim 1, and further comprising at least one opticalfiber positioned to receive the first and the second output beams. 7.The light source system of claim 1, and further comprising abirefringent prism positioned to receive the first and the second outputbeams and make the first and the second output beams parallel.
 8. Thelight source system of claim 7, and further comprising at least oneoptical fiber positioned to receive the parallel first and second outputbeams from the birefringent prism.
 9. The light source system of claim7, and further comprising a walk off prism configured to bring theparallel first and second output beams together into a combined beam.10. The light source system of claim 1, and further comprising anisotropic acousto-optic modulator (AOM) positioned either to receivesaid input beam generated by said beam source before said input beamreaches said anisotropic AOM, or to receive said first and second outputbeams generated by said anisotropic AOM.
 11. The light source system ofclaim 10, wherein the isotropic AOM is configured to change thefrequency of the first and second beam components by a first amount. 12.The light source system of claim 11, wherein the first amount is in therange of about 30 to 500 MHz.
 13. The light source system of claim 10,wherein the anisotropic AOM and the isotropic AOM are pre-aligned in asingle package.
 14. The A light source system comprising: a beam sourcegenerating a first input beam of light with first and second beamcomponents, the first component having a first linear polarization and afirst frequency, the second component having a second linearpolarization and a second frequency, wherein the first and second linearpolarizations are orthogonal; an anisotropic acousto-optic modulator(AOM) positioned to receive the first input beam, wherein the AOM isoperable to change the polarization and frequency of the first and thesecond beam components in response to a control signal, and therebygenerate first and second output beams corresponding to the first andsecond components, respectively; and wherein the AOM is configured toincrease the frequency of the first beam component by a first amount anddecrease the frequency of the second beam component by a second amount.15. The light source system of claim 14, wherein the first amount andthe second amount are each in the range of about 10 to 450 MHz.
 16. Thelight source system of claim 14, wherein the first amount and the secondamount are the same.
 17. A method of generating light beams, the methodcomprising: providing an anisotropic acousto-optic modulator (AOM);directing a first input beam into the AOM, the first input beamincluding first and second beam components, the first component having afirst linear polarization and a first frequency, the second componenthaving a second linear polarization and a second frequency, wherein thefirst and second linear polarizations are orthogonal; and applying acontrol signal to the AOM, thereby causing the AOM to change thepolarization and frequency of the first and the second beam components,and generate corresponding first and second output beams.
 18. The methodof claim 17, wherein the first input beam is generated by a laser. 19.The method of claim 18, wherein the laser comprises a Zeeman laser. 20.The method of claim 18, wherein the laser comprises a two-mode frequencystabilized laser.
 21. The method of claim 17, wherein the first outputbeam has the second linear polarization, and wherein the second outputbeam has the first linear polarization.
 22. The method of claim 17, andfurther comprising: directing the first and the second output beams intoat least one optical fiber.
 23. The method of claim 17, and furthercomprising: receiving the first and second output beams with abirefringent prism; and making the first output beam parallel to thesecond output beam with the birefringent prism.
 24. The method of claim23, and further comprising: directing the parallel first and secondoutput beams into at least one optical fiber.
 25. The method of claim23, and further comprising: bringing the parallel first and secondoutput beams together into a combined beam.
 26. The method of claim 17,and further comprising: providing an isotropic acousto-optic modulator(AOM) positioned either to receive said input beam generated by saidbeam source before said input beam reaches said anisotropic AOM, or toreceive said first and second output beams generated by said anisotropicAOM.
 27. The method of claim 26, and further comprising: applying acontrol signal to the isotropic AOM, thereby causing the isotropic AOMto increase the frequency of the first and second beam components by afirst amount.
 28. The method of claim 27, wherein the first is in therange of about 30 to 500 MHz.
 29. The method of claim 26, wherein theanisotropic AOM and the isotropic AOM are implemented in a singlepackage.
 30. A method of generating light beams, the method comprising:providing an anisotropic acousto-optic modulator (AOM); directing afirst input beam into the AOM, the first input beam including first andsecond beam components, the first component having a first linearpolarization and a first frequency, the second component having a secondlinear polarization and a second frequency, wherein the first and secondlinear polarizations are orthogonal; applying a control signal to theAOM, thereby causing the AOM to change the polarization and frequency ofthe first and the second beam components, and generate correspondingfirst and second output beams; and wherein the control signal causes theAOM to increase the frequency of the first beam component by a firstamount and decrease the frequency of the second beam component by asecond amount.
 31. The method of claim 30, wherein the first amount andthe second amount are each in the range of about 10 to 450 MHz.
 32. Themethod of claim 30, wherein the first amount and the second amount aresubstantially equal.
 33. An interferometer system comprising: a beamsource generating a first input beam of light with first and second beamcomponents, the first component having a first linear polarization and afirst frequency, the second component having a second linearpolarization and a second frequency, wherein the first and second linearpolarizations are orthogonal; an anisotropic acousto-optic modulator(AOM) positioned to receive the first input beam, wherein the AOM isresponsive to a control signal to change the polarization and frequencyof the first beam component, thereby generating a corresponding firstoutput beam, and change the polarization and frequency of the secondbeam component, thereby generating a corresponding second output beam;interferometer optics for generating a reference beam and a measurementbeam based on the first and second output beams; and an analysis systemfor determining movement information based on the reference beam and themeasurement beam.
 34. The interferometer system of claim 33, wherein thebeam source comprises a Zeeman laser.
 35. The interferometer system ofclaim 33, wherein the beam source comprises a two-mode frequencystabilized laser.
 36. The interferometer system of claim 33, wherein thefirst output beam has the second linear polarization, and wherein thesecond output beam has the first linear polarization.
 37. Theinterferometer system of claim 33, and further comprising an isotropicacousto-optic modulator (AOM) configured to increase the frequency ofthe first and second beam components; wherein said isotropic AOM ispositioned either to receive said input beam generated by said beamsource before said input beam reaches said anisotropic AOM, or toreceive said first and second output beams generated by said anisotropicAOM.
 38. The interferometer system of claim 37, wherein the anisotropicAOM and the isotropic AOM are implemented in a single package.
 39. Aninterferometer system comprising: a beam source generating a first inputbeam of light with first and second beam components, the first componenthaving a first linear polarization and a first frequency, the secondcomponent having a second linear polarization and a second frequency,wherein the first and second linear polarizations are orthogonal; ananisotropic acousto-optic modulator (AOM) positioned to receive thefirst input beam, wherein the AOM is responsive to a control signal tochange the polarization and frequency of the first beam component,thereby generating a corresponding first output beam, and change thepolarization and frequency of the second beam component, therebygenerating a corresponding second output beam; interferometer optics forgenerating a reference beam and a measurement beam based on the firstand second output beams; an analysis system for determining movementinformation based on the reference beam and the measurement beam; andwherein the AOM is configured to increase the frequency of the firstbeam component by a first amount and decrease the frequency of thesecond beam component by a second amount.
 40. The interferometer systemof claim 39, wherein the first amount and the second amount are equal.